The archaeological team here at Wairau Bar has been bolstered by the arrival of Hans Bader, a field archaeologist from Auckland, and some pretty impressive technology. One of the big problems about digging here is the sheer scale of the site – the Bar itself is 8 kilometres long, and although the main settlement we are interested in appears to be centred around the northern end, there’s still a lot of ground to cover – it’s at least 8 hectares in area (80,000 square metres, or about 12 rugby fields!) – and it may well be twice that size.
The main goal of what the archaeologists are doing is to get maximum information with a minimum of physical disturbance to this sacred site – we can’t just go around digging a bunch of holes willy-nilly – so where do you dig? And where do you not dig? (remember the last thing we want to do is disturb more graves). Back in the 1940’s researchers “pinpointed” (in the loosest possible sense of the word) areas of interest by running a plough over the site, and digging wherever they found artefacts or bones – including unfortunately the bones of tupuna, who if you remember were buried less than 50cm or so under the surface.
Obviously (and I hope it’s obvious) this is not a desirable method to use in 2009.
But we still have the problem of figuring out where we should place our 5X5 or 5X3 metre excavation sites among the vast expanses of flat, virtually featureless, ground here at the Bar.
Luckily Hans has arrived with solution to this problem. It’s called “fluxgate gradiometry” and it allows us to “see” under the ground and detect areas that have been disturbed in the distant past, like middens, hangi pits or fireplaces, even if there’s no sign of them on the surface. It’s all to do with the Earth’s magnetic field, and how it travels through different types of soil. The actual physics are complex and headache-inducing, but this is my non-specialist understanding how it works.
As you might know, planet Earth produces a magnetic field around itself, originating from inner core which is made of liquid iron :
The strength of this magnetic field changes, depending on where you are on Earth and how deep you are under it, since the field is generated by the core in the centre. Not only does it change depending on where you are standing, it also changes depending on what exactly it’s traveling through, and this is where Hans comes in. He has been surveying the site with a machine called a fluxgate gradiometer, which can detect minute changes in this magnetic field. Where someone has dug a pit or even lit a fire, the strength of the field will be different from the undisturbed ground around it, and these changes persist even if it’s been centuries since the hangi or midden was dug.
So Han measures out a 100 by 50 metre area, straps on his rig, and carefully walks up and down a rope line every 2 metres along the survey area, while the fluxgate machine measures what’s under his feet :
This is a closer look at the machine – the two vertical black poles are magnetic field detectors, which send data to the recorder in the middle :
When he’s finished each area, he plugs the data into his laptop, and ends up with a map of what’s UNDER the ground, all without disturbing a teaspoon of soil. (the only things that are disturbed are gorse bushes, and boxthorn – a b*stard of a plant that grows everywhere and has thorns that can go through the tyre of a 4 wheel drive – no kidding).
Here’s what he can reveal about what’s under the ground (click to embiggen) :
Pretty impressive, huh? As you can see, this method shows where hangi pits, fireplaces and midden have been dug 700 years ago, in a non-destructive, non-ploughing way. It’s not sensitive enough to detect individual artefacts or organic material, but thanks to Hans the other archaeologists get vital clues about where to excavate and get good results, without needlessly digging lots of useless holes. He’s still got a lot of ground to cover, but the fluxgate work is already showing some impressive results, and also may indicate that the settlement at Wairau Bar is much larger than we first thought.